Aluminum metal is produced industrially by electrolysis of smelter grade (or other) alumina in a molten electrolyte, using the well-known Hall-Héroult process. This process may be referred to herein generally as a smelting process. The electrolyte is contained in a pot comprising a steel pot shell, which is coated on the inside with refractory and insulating materials, and a cathodic assembly located at the bottom of the electrolytic cell. Carbon anodes extend into the above referenced electrolyte composed of molten cryolyte and dissolved alumina. A direct current, which may reach values of more than 500 kA, flow through the anodes, electrolyte and cathode generating reactions that reduce the alumina to aluminum metal, and that heat the electrolyte by the Joule effect to a temperature of approximately 960° C.
Emissions from the electrolytic cell are comprised of a number of gaseous and particulate constituents, also referred to as process gases, such as hydrogen fluoride (Fg) and particulate fluoride (Fp). The mechanics involved with the generation of Fg and Fp by the electrolytic cell include:                i. electrochemical hydrolysis of hydrogen (H) sources that react with the molten electrolyte (at ˜960° C.) forming gaseous hydrogen fluoride (e.g. structural hydroxyls (OH) in the alumina as measured by the Loss on Ignition (LOI 300° C. to 1000° C.), hydrogen in the carbon anodes);        ii. thermal hydrolysis of hydrogen (H) sources entering the electrolytic cell that react with electrolyte vapor (˜400° C.) escaping through the crust forming gaseous hydrogen fluoride (e.g. loosely bound moisture on the surface of alumina, moisture from ambient air drawn into the electrolytic cell, and incomplete calcination of gibbsite in the alumina as measured by the Moisture on Ignition (MOI 20° C. up to 300° C.));        iii. electrolyte vapor that condenses forming fine fluoride particulates; and        iv. particulate materials containing fluoride that are entrained in electrolytic cell process gases.        
Recovery of Fg and Fp are of primary importance to the environment and metal reduction cost. Total fluoride (Fg and Fp) emissions to the environment are from two principle sources; that being, fugitive emissions that escape the electrolytic cell hooding and gas collection systems, which are discharged by the pot-room roof ventilator in a thermal plume to the environment, and the residual fluoride emissions that are not adsorbed by the alumina and filtered in the injection type dry scrubbing system, which are discharged to the environment by an exhaust stack. The former being the dominate emission source typically in the order of 4 times that discharged by the dry scrubber stack measured in terms of total mass of fluoride (Ft=Fg+Fp) released per tonne aluminum produced (kg Ft/t Al).
In terms of emission containment and capture, removable side covers facilitate periodic carbon anode assembly replacement and form the electrolytic cell's hooding system for minimizing the release of fugitive (untreated) emissions directly to the pot-room and environment. The emission collection efficiency of an electrolytic cell is predominately dependent upon the hooding efficiency, defined as the ratio of open area to closed area of a theoretically sealed electrolytic cell superstructure. The electrolytic cell ventilation rate and related under-pressure created inside the hooding system are important to ensure efficient emissions capture. The hooding efficiency and ventilation rate determine the leakage of fugitive emissions to the pot-room environment from the electrolytic process through gaps at the seams of the superstructure side cover panels, around anode stem penetrations and end tapping doors when the electrolytic cell is closed or open for electrolytic cell maintenance and metal tapping. The ventilation rate from each electrolytic cell consists predominately of ambient air drawn into the electrolytic cell through gaps in the hooding system to ensure efficient capture of the process gases and pollutants. The ventilation capability (size) of the Gas Treatment Centre (GTC) is strongly influenced by this ingress of ambient air drawn into the electrolytic cell.
The net ventilation volume from the electrolytic cell is the sum of the process gases produced by the smelting process (typically less than 1% of net ventilation volume) and ambient air (typically 99% of net ventilation volume) drawn into the electrolytic cell through gaps in the hooding system. The process gas temperature varies indirectly with the process gas flow meaning that conventional smelting process systems with significantly reduced ventilation flow can theoretically generate process gas temperatures up to about 400 degrees celcius.
In addition to the fluoride emission rate from the pot-room roof being significantly greater in magnitude than that from the discharge stack of the injection type dry scrubbing system, fugitive (untreated) fluoride emissions released to the pot-room and environment are also considerably cooler than the residual fluoride emissions emitted from the stack of the injection type dry scrubbing system. The dispersion of emissions into the atmosphere for a given set of meteorological conditions is predominately driven by thermal buoyancy in the plume. Thus, the dispersion of cooler pot-room fugitive emissions tends to be considerably less effective than that for residual (relatively hotter) fluoride emissions from the dry scrubber stack.
Dry adsorption and chemisorption of gaseous fluorides onto the surface of fresh alumina followed by the recycle of the fluorinated alumina back to the electrolytic cell, as the feed material for an aluminum smelting process, is widely accepted as the best available technique for abating fluoride emissions from an electrolytic cell. The injection type dry scrubbing system uses a two-step integrated process; first adsorption followed by chemisorption of gaseous hydrogen fluoride onto the surface of smelter grade alumina, and then the disengagement and filtration of the alumina and particulate before releasing scrubbed gases (including residual emissions) to the environment.
The following description refers to FIG. 1, which is a schematic diagram illustrating a conventional smelting process system with a centralized Gas Treatment Center (GTC) 1.30 using injection type dry scrubbing outside of the electrolytic cell 1.31 and pot-room building 1.19. The process gases from the superstructure 1.1 of each electrolytic cell 1.31 are collected and conveyed to the centralized GTC 1.30 by a process gas duct 1.34. Fugitive (untreated) process gases not captured in each superstructure 1.1 by a conventional hooding system escape into the pot-room building 1.19 where they are then vented to the environment in an emission plume 1.22 through a roof gravity ventilator 1.20. The primary scrubbing of hydrogen fluoride occurs at the reactor 1.23 where fresh alumina 1.24 and recycled fluorinated alumina 1.25 are injected into the process gases from the process gas duct 1.34 and exit the reactor as semi-scrubbed process gases through a semi-scrubbed process gas duct 1.35. The mixture of the semi-scrubbed process gases and fluorinated alumina are separated by secondary scrubbing using one or more filters 1.26. Secondary scrubbing occurs at the filter cake on the outer surface of the filters 1.26. The recycled fluorinated alumina 1.25, injected at several times the fresh alumina 1.24 injection rate, improves the contact quality between the process gases and alumina resulting in better fluoride distribution in the fluorinated alumina 1.33 and a higher gaseous fluoride adsorption efficiency. The recycled, fluorinated alumina 1.25 also provides limited reserve scrubbing capability should the fresh alumina 1.24 be interrupted. It is, however, preferable to reduce, if not eliminate, the recycle rate (ratio of the recycled, fluorinated alumina 1.25 to fresh alumina 1.24) as high recycle rates are known to contribute to increased alumina attrition, scale formation, energy consumption, system abrasion and dust load on the filters 1.26. Contact time between the alumina and hydrogen fluoride in the process gases for primary scrubbing is measured in terms of seconds. The total average contact time between the alumina and hydrogen fluoride in the process gases for primary scrubbing and in the semi-scrubbed process gases for secondary scrubbing, when considering recycle rate and secondary reaction time at the surface of the filters 1.26, is measured in terms of one to two hours. The scrubbed process gases 1.29 and residual fluorides are vented to the environment by exhaust fans 1.28 and a stack 1.32. The fluorinated alumina 1.33 is typically stored in a fluorinated alumina bin 1.27 and then conveyed back to each electrolytic cell 1.31 by a fluorinated alumina conveyor 1.18 where it is stored in fluorinated alumina superstructure bins 1.21 as feedstock for each electrolytic cell 1.31.
For injection type dry scrubbing systems several factors are paramount for achieving efficient hydrogen fluoride adsorption on the surface of the alumina, and even distribution of fluoride (Fg+Fp) in the fluorinated alumina recycled back to the electrolytic cells, specifically:                i. contact quality in terms of the intra-particle diffusion resistance between the hydrogen fluoride gases and alumina particles at the injection site;        ii. process gas temperature during the adsorption process; and        iii. an equal mass of fresh alumina reacting with an equal mass of hydrogen fluoride in the process gases between all operating filter compartments.        
Depending on the electrolytic cell operating current, electrolytic cell heat balance and cover material (crust) maintenance practices, the temperature of the process gases exhausted from conventional electrolytic cells typically varies between 100° C. to 140° C. above ambient temperature. Due to heat loss from the process gas collection ductwork, the process gas temperature typically enters the GTCs (without additional cooling) between 85° C. to 125° C. above ambient temperature. A common practice employed today for injection type dry scrubbers is to limit the gas temperature entering the GTC to 115° C. to 125° C. to enhance the adsorption of hydrogen fluoride on to the surface of alumina. The corresponding temperature of the fluorinated alumina discharged from conventional dry scrubbers and stored in the bins of an electrolytic cell's superstructure, which is then dosed into the molten electrolyte, is typically 10° C. to 20° C. below the process gas temperature entering the dry scrubbers.
In addition, International Publication WO 2008/024931 describes emissions test data for injection type dry scrubbing in a smelting process that confirms a strong correlation between the amount of gaseous fluoride in scrubbed process gas emissions after injection type dry scrubbing and the process gas temperature entering the GTC. This correlation has led to lowering the process gas temperature before the dry scrubbing process by direct or indirect cooling methods in order to reduce the gaseous fluoride in scrubbed process gas emissions after injection type dry scrubbing. As a result, conventional injection type dry scrubbing systems do not address abating the formation of fluoride at the source in an electrolytic cell because the amount of hydrogen entering the electrolytic cell with the fluorinated alumina returned from the injection type dry scrubbing systems, in the form of free moisture, increases as the process gas temperature entering the dry scrubbing process decreases.
The next generation of potlines will exceed 460 electrolytic cells thus, extending the length of the pot-rooms to over 1,300 meters. In addition to longer potlines, electrolytic cell amperage has and will continue to exceed well beyond 500 kA. As a consequence, the energy released to the process gases has and will continue to increase the process gas exhaust temperature, thereby potentially reducing the adsorption efficiency of gaseous fluoride on the surface of the alumina and eroding adsorption efficiency of injection type dry scrubbing systems if suitable countermeasures to cool the gases are not included in the GTC design. The injection type dry scrubbing systems used on new electrolytic cell potlines have followed an economies of scale approach resulting in an ever increasing dry scrubber system size and process gas conveyance distance. This approach further increases the capital and operating costs at little to no incremental benefit, which is now at the point of diminishing returns. Constructing large GTC systems (and related ancillaries) in the courtyard between pot-rooms also competes with and interferes with the construction of the smelting process plant housed inside the pot-rooms. The congestion in this area creates inefficiency and raises the probability of an incident putting people, equipment and the execution schedule at ever increasing risk.
Existing emission control systems, configured outside of the electrolytic cell and pot-rooms, often require additional investment to upgrade or replace non-compliant environmental control systems as part of the plan to incrementally increase the operating current of an aluminum smelting process to incrementally increase metal production. The additional investment required for environmental compliance is to the detriment of the upgrade project's economic viability.
Economies of scale combined with relatively low-cost energy, vital to this energy intensive industry, have driven the capacity of the largest aluminum smelter plants to well over one million tons of annual capacity. Compliance with ambient air quality concentration (μg Fg/m3) standards at ground contact for gaseous fluoride and sulphur dioxide emissions becomes a significant challenge for such large process plants. The modern smelters operating with over a million tons of annual aluminum smelting capacity require and use additional emission abatement equipment and systems to comply with the prevailing regulatory requirements, often doubling the investment and operating cost of emission abatement systems and in some cases waste water outfalls to the sea.
The formation and accumulation of hard grey scale in the conventional injection type dry scrubbing system can, if not properly managed, severely diminish emission abatement performance of the GTC, and in some cases cause the shutdown of one or more scrubbing modules or ventilation fans for maintenance. The scale formation rate of alumina is a function of flow turbulence, the presence of fluoride and bath species, the presence of minus 20 micron particles, and the presence of moisture—if any one of these four elements are removed and or diminished, scale formation would respectively be eliminated or dramatically reduced.
Purity of aluminum metal produced by electrolysis in a conventional electrolytic cell is, to a great extent, a function of alumina quality fed to the electrolytic cell. The quality of the fluorinated alumina, in terms of impurities, is a function of the impurities collected from all operating electrolytic cells connected to the GTC gas collection system. Metal purity and its variation from any one electrolytic cell are negatively impacted by the poorest performing electrolytic cells connected to the same GTC.
Electrolytic cell operating efficiency is, among other things, a function of the alumina quality fed to the electrolytic cell. The quality of the fluorinated alumina generated by the conventional injection type dry scrubbing system, in terms of the fines content, temperature and moisture content is a function of the GTC design and its operating parameters. The conventional injection type dry scrubbing system can negatively impact alumina quality in terms of:                i. Fines content: Increasing the fines content of fresh alumina due to particle attrition created by material handling and injection (including alumina recycle). Increasing the fines content impedes alumina dissolution into the molten electrolyte, increasing the potential for what is known in the industry as “slugging;”        ii. Temperature: Relatively cool alumina temperature fed to the electrolytic cell, as compared to the molten electrolyte temperature, requires additional energy to heat the feed material;        iii. Moisture: Free moisture (water) entering the electrolytic cell with the alumina feed material, measured in terms of MOI and to a lesser extent LOI, requires energy to drive off the moisture and leads to what is known in the industry as the “volcano effect” due to the flash vaporization of the moisture when alumina is fed into the cryolyte, which impedes alumina dissolution.        
To date, no viable alternatives (to conventional GTC configurations) are known for reducing the life cycle cost for recovering fluorides from the electrolytic cell process gases at the same, or better, adsorption efficiency than is achieved today.